Searching for mobile data offloading solutions has been topical in recent years. In this paper, the authors present a collaborative WiFi-based mobile data offloading architecture - Metropolitan Advanced Delivery Network (MADNet), targeting at improving the energy efficiency for Smartphones. According to their measurements, WiFi-based mobile data offloading for moving Smartphones is challenging due to the limitation of WiFi antennas deployed on existing Smartphones and the short contact duration with WiFi APs. Moreover, their study shows that the number of open-accessible WiFi APs is very limited for Smartphones in metropolitan areas, which significantly affects the offloading opportunities for previous schemes that use only open APs.